FIELD OF THE INVENTION
The invention relates to a circuit configuration for generating an internal supply voltage, with which integrated circuits can be operated.
As the scale of integration in integrated circuits increases, the dimensions of the integrated components become smaller and smaller. It is highly important that the space requirement per memory cell be slight, especially in semiconductor memories, in which the storage capacity and thus the number of memory cells are being increased more and more.
However, the increased scale of integration has meant that the electrical field intensity at the individual components of the integrated circuit, for instance at the gate oxides of transistors, is greater in comparison with memories having a lesser scale of integration. Thus the stress exerted on the components also rises, leading to a growth in failure rates. In order to avoid that, the cell fields of semiconductor memories are operated with an internal supply voltage. As a rule, it is below the external supply voltage with which the external circuit located outside the cell fields is operated. For instance, in the case of the cell field, the voltage of 5 V of the outer circuit is reduced to the internal supply voltage of 3.3 V. Various circuits for reducing the voltage are known.
The dependency of the service life of the cell field on the internal supply voltage being applied and on the resultant electrical field can be exploited in a so-called burn-in test. In it, the cell field is operated at a higher voltage than the internal supply voltage used for proper operation. The resultant failures of the memories make quality control possible.
Only the external supply voltage can be applied to the semiconductor memory from outside. The internal supply voltage, which should be as constant as possible and as independent as possible of external interfering factors, is generated by a voltage generator provided specifically for that purpose. Since the internal supply voltage is regulated to a certain value by the voltage generator, increasing the external supply voltage does not at the same time lead to an increase in the internal supply voltage. It is therefore not possible to perform the burn-in test with conventional voltage generators.
A voltage generator which furnishes a regulated, constant internal supply voltage as long as the external supply voltage is below a certain value, is known from German Published, Non-Prosecuted Patent Application DE 42 26 048 A1. If the external supply voltage exceeds that certain value, then the internal supply voltage rises with the external supply voltage. That is attained due to the fact that either a constant comparison voltage or the external supply voltage, depending on whether the external supply voltage is below or above that certain value, is supplied to a closed control loop that generates the internal supply voltage.
The disadvantage of that voltage generator is that a relatively complicated and expensive device is needed for the burn-in test in which semiconductor memories with that voltage generator are to be tested. That is because in order to expose the semiconductor memory to a defined stress, the external supply voltage must be kept at a very specific value, which to that end should be as constant as possible.